Open your map, and press LB/RB to go to the online contracts. Go to the top right and select the Street Sweeps online missions (Note, you must first have done at least the Fading Signal mission to unlock the game type).
Once you find someone, both go to one of the orange street sweep missions on the map. Finish it, and that's 1 of your ten. They also count towards the separate achievements for the street sweeps.